The example you provide shows a page of spatially arranged addition problems, with missing digits appearing as squares. Section 58 of the Nemeth Code prohibits the use of shape symbols in work arranged spatially for computation. Only the general omission symbol may be used (dots 123456). As far as I know, Guidelines and Standards for Tactile Graphics does not prohibit the use of the general omission symbol in K-3 materials. Note also that the number of general omission symbols to be used must be the same as the number of omission signs which occur in print. In your example, item 5 will show two general omission symbols in the missing answer.